Not so true on 3/4 and 1 ton pickups.
The axle on my pickup is the same axle, and same brakes, as the one used on a DRW. The GAWR has a difference of 3150lbs, but only a GVWR difference of 2300lbs.
Ford even discusses that brakes are sized to the GVWR in their towing brochure. "Towing vehicle’s braking system is rated for operation at GVWR"... which on any HD pickup is less than the sum of front and rear.
On my TT the 5200lb axle has the same brakes as a 6K, 7K, and some 8K lbs axles. The brakes are not the limiting factor there, even the bearings are the same as the 7K axle, but a different tube/hub are on the 7K (and some 6K have the 8 lug hub). Brakes are not a determining factor on my trailer axles.
This is why one must research their truck/trailer if they plan to exceed GVWR.
All my research on pickups has lead me to the conclusion than brakes are based on GVWR, not FAWR+RAWR... The only time I have found it to not be true is looking at multiple GVWR's with the same brakes.. i.e. a 3/4 Ram has the same brakes as a DRW Ram in my year... or some cab configurations change GVWR for the same components. So the brakes are designed for the highest rated application.
As far as DOT is concerned they don't care, at least here in Idaho, about any GVW or GAW... simply what I have registered for and what tires I have (and under 20K/axle). Beyond that it's up to me to be smart about loading. I know I have 9K worth of tires on my front axle, and I also know the axle itself can't go that high.
Looking thru specs there isn't a single 3/4 or 1 ton SRW truck I can find that isn't tire limited. That takes everything into account. Most 1/2 tons are axle limited these days... in years past they have been tire limited but with all the 17-22" rims out there the tires hold a little more, even as LRB.
Having pulled for a living and delt with dot issues in multi states...let me just say not much there I would agree with.
